1. Introduction to Food Engineering
Psychrometrics
Introduction to Food Engineering

2. Psychrometrics
Thermodynamic properties of gas-vapor mixtures (air-water)

3. Properties of Dry Air Composition N2 78 %, O2 20.9 %
Standard dry air MW =
Gas constant for dry air
Ra = m3.Pa/kg.K

5. Properties of Dry Air Specific Volume Ra = gas constant
TA = absolute temperature (K)
Pa = partial pressure of dry air

6. Properties of Dry Air Specific Heat (Cpa) Enthalpy (heat content)
At 1 atm ( kPa), T = - 40 – 60 C
Average value kJ/kg.K
Enthalpy (heat content)
Reference 1 atm, 0 C

7. Properties of Dry Air
Dry Bulb Temperature
Indicated by sensor

8. Properties of Water Vapor
Moist air = dry air + water vapor
Vapor in the air is superheated steam at low pressure & temperature
Moist air is clear or foggy
MW of water =
Gas constant for water vapor
Rw = m3Pa/kg.K

9. Properties of Water Vapor
Specific Volume of Water Vapor
Below 66 C vapor follows ideal gas law

10. Properties of Water Vapor
Specific Heat of Water Vapor
Within –71 to 184 C
Cpw = 1.88 kJ/kg.K
Enthalpy of water vapor
Ta = dry bulb temp

11. Properties of Air-Vapor Mixtures
Gibbs-Dalton Law
Up to 3 atm air-water mixtures follow perfect gas laws
PB = barometric (total pressure) of moist air (kPa)

12. Dew-Point Temperature
Water vapors in air = steam at low pressure
Saturation temperature = dew point
Obtained from steam table at partial pressure exerted by water vapor
Below this temp, condensation of moisture

13. Humidity Ratio
Humidity Ratio (Moisture Content) or specific humidity, W
(Kg water / ky dry air)

14. X = mole fraction

15. Relative Humidity
Relative to maximum amount of moisture at dry bulb temperature

16. Relative Humidity Density
Density of water vapor/density of saturated vapor at dry bulb temp of air

17. Humid Heat Of air-water vapor mixture
Heat required to raise temp of 1 kg dry air + water vapor by 1 K
(kJ/kg dry air . K)
W = humidity ratio

18. Specific Volume
Volume of 1 kg dry air + water vapor

19. Adiabatic Saturation of Air
Evaporation of water by sensible heat of entering air

21. Wet-Bulb Temperature Psychrometric wet bulb temperature
Movement of air
Thermodynamic wet bulb temperature

22. The Psychrometric Chart

23. Example
An air-vapor mixture is at 60 C dry bulb temp and 35 C wet bulb. Determine relative humidity, humidity ratio, specific volume, enthalpy and dew-point temp.
RH = 20 %, W = kg/kg
Enthalpy = 129 kJ/kg dry air
Specific volume = 0.98 m3/kg dry air
Dew-point temp = 29 C

24. Use of psychrometric chart
to evaluate complex air-conditioning processes
Heating or Cooling of Air
Humidity ratio constant

25. Example
Calculate the rate of thermal energy required to heat 10 m3/s of outside air at 30 C dry bulb temp and 80 % RH to a dry bulb temp of 80 C
Mass flow rate = vol flow rate/ specific vol

26. Mixing of Air
Inverse proportion

27. Drying Adiabatic saturation process
Heat of evaporation is supplied only by drying air
Dry bulb temp decreases, enthalpy constant ie. constant wet bulb
Humidity ratio increases (gain moisture)

29. Example
Heated air at 50 C, 10 % RH is used to dry rice. Air exits under saturated condition. Determine amount of water removed per kg of dry air.
W1 = kg/kg
Follow constant enthalpy line
W2 = kg/kg
Moisture removed = kg/kg